1. Field of the Invention
The invention concerns an X-ray tube, especially for X-ray examination of the breast and, in particular, it concerns an anode target made of molybdenum.
With an X-ray tube, the X-radiation is obtained under the effect of an electronic bombardment of a target borne by the anode or formed by the anode itself. On the target, a small surface is subjected to electron bombardment and forms the source of X-radiation. The characteristics of the X-radiation depend on the characteristics on the incident electron beams and the nature of the material forming the target.
Molybdenum targets are commonly used in the anodes of X-ray tubes designed for X-ray examination of the breast.
The usefulness of a molybdenum target, in X-ray examinations of the breast, lies especially in the fact that the energy spectrum of the X-radiation emitted by molybdenum is particularly suited to the specific nature of such examinations. For, the breast has low X-ray absorption characteristics, and the anomalies sought have density levels which are very close to those of surrounding tissues. The contrast presented, between these anomalies and the surrounding tissue, in an X-ray picture is considerably improved when the X-radiation used has a narrow energy band containing the characteristic lines displayed by molybdenum.
Molybdenum targets are commonly obtained by sintering molybdenum powder. Most often, it is the anode itself that is made by sintering molybdenum powder, the target being formed by a part of the anode. The anode may be of the fixed anode or rotating anode type. Other standard methods such as, for example, vapor phase chemical deposition or, again, electrolytic deposition enable the depositing of a layer of molybdenum on the anode in order to form a target on the entire surface of the anode or on a part of this surface, along a focal track for example, in the case of a rotating anode.
The repeated electron bombardments to which the molybdenum target is subjected create thermo-mechanical strains in this target, causing the molybdenum to form cracks. The cracks increase in number and size with the number of exposures. These cracks reduce the X-ray transmission efficiency of the X-ray tube. This can be explained by the fact that electrons which fall in the cracks create X-radiation, a large proportion of which is absorbed in the anode itself.
The fall in X-ray efficiency leads, particularly, to the following two drawbacks which are particularly important in the case of X-ray examinations of the breast.
1.degree. A deterioration in the quality of the image which can be explained, firstly, by an increase in kinetic fuzziness or movement fuzziness, which is due to the increase in exposure time made necessary by the fall in X-ray efficiency and, secondly, by the non-homogeneity in the density of the picture, due to an accentuation of a phenomenon of variations in X-ray efficiency in the field. PA1 2.degree. An increase in the dose received by the patient, owing to deviation from the so-called relationship of reciprocity between the film/screen pair used in X-ray examinations of the breast: for, the non-reciprocity of the receiver (film/screen) makes the quantity of photons needed to achieve maximum density of the film increase when the exposure time increases.
Thus, for example, in a direction neighbouring that where the X-radiation is at its maximum, this intensity is reduced by about 20% after three months of normal operation of the X-ray tube, and it is reduced by about 40% in a plane neighbouring the anode plane.
The present invention concerns an X-ray tube with a fixed or rotating anode which is particularly, but not exclusively, designed for X-ray examination of the breast, said anode having a molybdenum target which does not have the above-mentioned problems of cracking or, if at all, then at the end of a far longer operating time and to a much smaller degree than with a prior art molybdenum target
This is obtained by doping of the molybdenum. We feel that this doping has the effect of reinforcing the grain association and the molybdenum grains themselves, and of making the molybdenum more elastic.